1. Field of the Invention
The present invention generally relates to the authentication and non-repudiation of communications between a transmitter and a receiver and, more particularly, to an apparatus for the authentication and non-repudiation of transmitted documents using a digital signature. The invention is particularly useful in the authentication and non-repudiation of documents transmitted between facsimile machines by analog telephone line, but may be advantageously used to authenticate and prevent repudiation by the sender of other analog messages including, but not limited to, telephone conversations, analog video and the like.
2. Description of the Prior Art
Digital signatures have been used for some time on digital documents such as electronic mail to provide the two main functions of an ink signature on a paper document, namely: (1) "authentication" or proving to the receiver that the document was created by the purported sender and has not been modified in transit, and (2) "non-repudiation" or enabling the receiver to prove to a third party, such as a judge, that the document was created by the purported sender. This latter ability prevents the sender from repudiating a genuine document, such as a promise to pay, by falsely claiming that it is a forgery created by the receiver.
Most digital signature schemes use a public/private key system to provide authentication and non-repudiation for transmitted data. See for example W. Diffie and M. Hellman, "New Directions in Cryptography", IEEE Transactions on Information Theory, IT-22, pp. 644-645 (November 1976), and R. L. Rivest, A. Shamir and L. Adleman, "A Method for Obtaining Digital Signatures and Public-Key Cryptosystems", Communications of the ACM, 2, No. 21, pp. 120-126 (February 1978). Digital signatures obtained with a public-key algorithm can be validated by anyone knowing the public-key of the sender.
Ordinary documents transmitted by facsimile lack the verifiable authenticity of paper documents in two respects. First, they can be forged by third parties claiming to be the purported sender, or be subjected to undetectable modification in transit. Second, a genuine document can later be repudiated by the actual sender, who may falsely claim that the document is a forgery created by the receiver.